Energy translation utilizing pyroelectricity



Oct. 20, 1942. L. J. SIVIAN, 9,

ENERGY TRANSLATION UTILIZING PYROELECTRICITY Filed Aug. 31, 1939 2 S'hegts-Sheet 2 air 24! naa wvsu TOR L .J. 5/ WAN ATTORNEY Patented Get. 20, W42

ENERGY TRANSLATION in; I l

PYROELECTRKCH'EY Leon J. Sivian,

East Orange, N. 3., asslgnor to Bell Telephone Laboratories, Incorporated, New York, N. K, a corporation of New York Application August 31, 1939, o'erlal No. 292,902

8 Claims.

This invention relates to methods of and apparatus for utilizing pyroelectricity in energy translation systems.

It has long been known that certain substances, 1

when warmed or cooled. exhibit an electrification which manifests itself as a change in electrical polarization of the substance or separation of its.

served in studies of a large number of crystalline substances, a list of some of which is given at Table 3, page 209, vol. 6, of the International Critical Tables, published in 1929 for the National Research Council by McGraw-Hill Company. Among these substances are tourmaline, lithium sulphate and lithium selenate.

The general nature of pyroelectricity may be readily demonstarted by subjecting a crystal of tourmaline suspended by an electrically insulating thread to a heating atmosphere. One end of the crystal will be found to be positively electrified and the other negatively electrified. When .such a pyroelectric crystal is cooled its electrical polarity becomes reversed from that produced by heating; become negative and that which was negative has become positive. If the crystal be subjected to alternate heating and cooling an alternating electrical polarization will ensue.

An object of this invention is to take advantage positive and negative charges. This 'phe-' 'nomenon known as pyroelectricity has been obthe end which was positive now has trically insulated from each other. A circuit leading from the electrodes of the pyroelectric plate may be connected to an amphfier with the output circuit of which an indicator is electrically assoclated. Consequently, incidence of the wave energy beam upon the pyroelectric plate may be made evident by the indicator.

Sound waves in air may be allowed to fall directly upon the pyroelectric plate or may pass through a diaphragm to a confined atmosphere in which the pyroelectric plate is positioned. The

adiabatic compression and rarefaction of the atmosphere immediately surrounding the plate give rise to a fluctuation in its temperature which is partially communicated to the pyroelectric plate thus causing a corresponding fluctuationof its temperature and a consequent alternating elecof the pyroelectric phenomenon in order to translate into electrical energy, impulses or trains of heat waves or of any wave energy which are capable of producing temperature variations in pyroelectric material.

Another object of the invention is to translate radiant energy into electrical energy by means. of pyroelectric apparatus which may at the same time be very rugged and extremely sensitive. An additional object of the invention is to improve the eifectiveness of a pyroelectric translatlng device for converting hert bearing waves into corresponding electrical waves.

A further object of the invention is to provide a sensitive detector for radiation energy of electromagnetic character in the frequency ranges commonly designated as heat and, light.

In accordance with one embodiment of the invention, a plate of a pyroelectric material is suspended in a medium traversed by beams of wave energy which'on incidence upon the material cause its temperature to change. The resulting electrification of the plate gives rise to charges upon conducting electrodes or coatings nection with the accompanying drawings in I placed in close association with the plate but electrical polarization at the electrodes associated with the plate. In fact the principle of the invention is applicable to any sort of systemijin which a disturbing or impelling force can set up an alternating temperature in a pyroelectric ele ment thus giving rise to a corresponding alternating pyroelectric charge between the associated electrodes.

Inasmuch as the pyroelectric phenomenon is dependent upon heat transfer to and from the pyroelectric element it is necessary in order to make the pyroelectric element sensitive to reduce its thermal inertia as far as possible. This may be efiectlvely accomplished by reducing the thickness of the pyroelectric element to very small dimensions as for example of the o der of .005 of an inch. A tourmaline plate having a thickness of .0015 of an inch has been found quite effective.

It is also advantageous in cases where the varying heat by which the temperature of the pyroelectric element is altered is conveyed by a gaseous medium to choose such a gas as will most readily conduct heat to or from the crystal. For this purpose hydrogen and helium are markedly superior to air.

If the surfaces of the pyroelectric plate be blackened to convert incident radiant energy into heat in the manner well known, the pyroelectrid operation may be utilized to enable the system to serve as a photoelectric cell or radiation detector.

The invention may best be understood by reference to the following specification taken in conwhich Fig. 1 shows a circular disc of pyroelectric material;

Fig. 2 is another view of the disc of Fig. 1,

llustrating the electrical coatings on its major aces;

Fig. 3 illustrates schematically a system for responding to sound waves and converting their energy by pyroelectric action to corresponding electrical waves;

Fig. 4 shows a modification in which the microphone element of Fig. 3 is positioned in an enclosed atmosphere of helium or hydrogen;

Fig. 5 shows a pyroelectric element having coatings similar to the element of Figs. 1 and 2 and with an additional black absorbing coating on one of its faces for converting radiant energy into heat;

Fig. 6 discloses a system responsive to radiant light or heat employing the detecting element of Fig. 5; and

Figs. 7 and 8 disclose systems employing a pyroelectric element in conjunction with a Helmholtz resonator to respond to incoming sound waves.

Referring to Fig. -1, a disc I of tourmaline is cut from the virgin crystalline material in such a manner as to have its major or parallel plane faces extending perpendicular to the optical or Z axis of the crystalline material. The disc is preferably made very thin as, for example, of the order of .0015 to .005 of an inch in thiclmess. As shown in Fig. 2. its major faces are provided with coatings 2 and 3 of conducting material such as aluminum or platinum which may be applied according to the methods now well known in the piezoelectric technique.

If such a crystal of tourmaline as that of Figs. 1 and 2 with its electrically conducting coatings or electrodes be subjected to an increase of temperature d0? 0., its major faces develop equal and opposite charges amounting to d? electrostatic units per unit area. If, on the other hand, the crystal be cooled by (10 C., the polarization or charge developed is also 411? electrostatic units per unit area but with an opposite polarity to that developed by imrease of temperature. The

ratio do is known as the pyroelectric coefficient. At room temperature of about 300' K.

has a magnitude of the order or 1.2. v

The system of Fig. 3 makes'use of the pyroelectric disc of Figs. 1 and 2 in a system for detection of sound waves. The element I is preferably supported by lead wires 4 and 5 soldered to its opposite electrodes within an electrically shielded or screened casing 6, the apertures of which are sufficiently large to enable sound waves to freely pass through the casing. The adiabatic compression and rarefaction of the' atmosphere within the casing 6 in consequence of translation of sound waves produces a'variation in its temperature and, also, therefore, in the temperature.

in the element l exposed to the atmosphere. The corresponding fluctuating electrical polarization developed upon the conducting electrodes 2 and 3 of the element l enables the device to act as a microphone or a detector of sound waves. The leads 4 and. 5 areconnected respectively to the inner and outer conductors of a coaxial conductor system 1 leading to the input terminals of an electron discharge device 8 within a shielded container 9. Various mechanical and electrical details of the apparatus are omitted to simplify the drawings. It will be readily understood, however, that the fluctuating potentials developed between the opposite electrodes oi the element I will be amplified by the device 0 and indicated by the responsive'device l0 in its output circuit. The electroresponsive device l0 may be a sensitive meter of any type or it may be -an electroacoustic device such as a telephone receiver or loud-speaker.

Fig. 4 illustrates a system for translation of sound waves into electrical wave energy in which the pyroelectric element l is mounted in a hermetically sealed container I! provided with a flexible diaphragm member l3 capable of transferring sound wave pressures from the outer atmosphere to the atmosphere within the chamber. The pyroelectric element l.is electrically associated with a coaxial system I in the same manner as in thedisclosure of Fig. 3. The hermetically sealed chamber 12 not only protects the pyroelectric element from moisture and the accumulation oi dust particles but, also, enables the atmosphere immediately surrounding the element l to be made such that its density, speciilc heat and heat conductivity result in a greater temperature variation in the pyroelectric crystal for a given pressure amplitude in the atmosphere. The chamber preferably contains an atmosphere of hydrogen or helium to improve the heat transfer to the pyroelectric element.

Fig. 5 discloses a modification of the pyroelectric device shown in Figs. 1 and 2 to adapt the device for reception of heat-bearing electromagnetic beams. For that purpose, a device similar to that of Fig. 2 is coated with a black absorbing coating ll of a type effective to convert incident beams of short electromagnetic waves into heat.

It is to be understood that any radiation which can be converted into heat by absorption such as electromagnetic waves of radiant heat or infra-red rays may be employed.

. Fig. 6 illustrates a system utilizing the device of Fig. 5 in which the pyroelectric element is enclosed in a sealed chamber I5 which may be exhausted to a very low pressure. The chamber llis provided with a window l6 of a material transparent to the light or heat wave or other radiant energy beams. Accordingly, beams of radiant energy passing through the window and falling upon the coating ll of the pyroelectric element are converted into heat energy to vary the temperature of the pyroelectric element. Preferably, the exhaustion of the chamber is carried to a highdegree for'the reason that the presence of air or other gas would, to an extent. absorb heat from the incoming radiant energy and would also serve to conduct heat away from the pyroelectric element thus causing the amplitude of alternating temperature of the-element to be lower than it would otherwise be. At a point in the path of the incoming beam which may be close to the window ii, if desired, or may be at a remote transmitting station, a rotary interrupting device I1 is provided with vanes which traverse the path of the light beam thus momentarily intercepting the beam. The interrupter I1 is preferably driven by an electric motor i8 supplied with energy from alternating current mains IS. A variable controlling element 20 connected in the current supply circuit enables the speed of the motor II to be varied as desired or even to be changed in accordance with code impulses. It follows that the beam of incident radiation may at times undergo intera fundamental distinction between the operation of a pyroelectric element and that of the wellknown piezoelectric element. Since piezoelectric electromotive forces are generated only as a result of pressures or pressure variations, the pyroelectric element I which is enclosed in the highly exhausted chamber It of Fig. 6 and is therefore free from pressure is not subject to forces which will cause it to generate piezoelectric electromotive forces. It will therefore generate pyroime instruments is not known with certainty, it is believed to be substantially as follows: The

1 heat generated in the wire 32 sets up an air cirelectric electromotive forces wholly free from piezoelectric electromotive forces.

The extreme case Just considered is particularly easy to grasp. In reality the distinction um tartrate tetrahydrate) immersed in an atmosphere and subjected over its entire surface to the same pressure from the surrounding medium generates no piezoelectric charge. It, therefore. the element I of Fig. 3 consists of Rochelle salt no piezoelectric electromotive force results from the variation in pneumatic pressure. However, an alternating pyroelectric charge is generated in Rochelle salt as'a result of the adiabatic temperature alternations accompanying a sound wave in air. It'is otherwise in the case of tourmaline, the quite different piezoelectric system of which permits a varying pneumatic air pressure to generate a piezoelectric charge. It will be apparent, however, that the mere fact that under certain conditions some substances may generate both piezoelectric and pyroelectric electromotive forces is no genuine indication of the general inseparability of the two-effects. It will be appreciated that in the aspects Just disc ed the performance of the structure of Fig. 4 i: similar to that of Fig. 3.

Another instance of a purely pyroelectric apparatus is disclosed in Figs. 7 and 8 in which a Helmholtz resonator at, the natural rmonance frequency of which corresponds to the frequency of an incoming sound wave to be indicated or detected, is provided at its throat portion it with an element 24 of pyroelectric substance having electrodes 25 and 2% which are connected by the coaxial line 21 to the input of an amplifying or detecting device 28, the output circuit of which includes a current indicator 2!! and the primary winding of a transformer 30 connected to a loudspeaker or other sound reproducer 35. Within the Helmholtz resonator 22 is a heating coil 32 in circuit with an external source 33 of heat electromotive force and a rheostat 36. Although the precise theory of operation of such "hot wire" culation with a central hot current upward and a cool peripheral downward current in the neck 23. The central hot current raises the temperature of the element 25, and the larger that current the higher the temperature of element 25. The oncoming sound wave imparts an alternating velocity to the air in the neck 23 as a whole, and, in particular, it accelerates and decelerates the velocity of the unidirectional central air current. The result is an alternating component in the temperature of the crystal, and an alternating electric polarization with it. Consequently it is possible to utilize the energy of the incoming sound waves to initiate varying pyroelectric electromotive forces of the same frequency in the circuit of the amplifier 28.

Fig. 8 discloses a modification of the apparatus of Fig. 7 in which the pyroelectric crystal is surrounded by a heating coil 36. The transfer of heat from the coil 36 to the pyroelectric element M will accordingly be modulated by the alternating current air flow in the throat portion of the resonator.

What is claimed is:

1. An apparatus for responding to electromagnetic wave energy of such wave-length as to convey heating energy comprising an evacuated container having a window pervious to radiant heat bearing waves, a thin element of pyroelectric material having electrodes upon which pyroelectric charges developed in the element may be impressed, means for supporting the element in front of the window and with its thinnest dimension extending in the direction of travel of the radiant waves, means for setting up intermittent variations in incoming radiant wave energy whereby the pyroelectric device is caused to yield a periodic electric response and means for varying the temperature of the pyroelectric element in consequence of the incidence of electromagnetic waves.

2. A hermetically sealed chamber of electrically conducting material having a window transparent to heat-bearing rays, a pyroelectric element within the chamber having electrically separated conducting electrodes, means for supporting the pyroelectric element comprising two conducting leads integrally connected electrically to the respective electrodes, one lead being elsetrically connected to the chamber and the other insulated therefrom to enable pyroelectric E. M. F.s. developed by the element to be applied to an external circuit connected between the insulated lead and the chamber, the supporting leads being so positioned as to cause the element to present a major surface to a beam of rays entering the window and incident upon the surface and a coating of heat-absorbing material on a portion of the surface of the element exposed to the rays whereby upon incidence of the rays a pyroelectric charge is impressed upon the elec-, trodes.

3. A hermetically sealed chamber, a window in one wall of said chamber to permit heat-bearing rays to pass into the chamber and a. pyroelectric element having conducting electrodes mounted within the chamber in position to intercept the heat-bearing rays, the pyroelectric element comprising a plate of tourmaline of the order of .0015 inch in thickness and the mounting supporting it in such position that a beam of heat bearing rays incident through the window strikes the element in the direction of its thickness to permit rapid heating and cooling of the entire mass, and a heat absorbing coating superposed on the electrode on the face of the element presented to the window to facilitate heatenergy transfer whereby upon incidence of the rays 8. pyroelectric charge is impressed upon the electrodes and upon diminution or withdrawal of t e rays heat energy is rapidly radiated from the ele ment tothe space beyond the window.

4. A pyroelectric element comprising a flat plate of tourmaline having its'parallel flat faces perpendicular to the optical axis of the crystallinematerial, said plate having a thickness of the order of .0015 to .005 inch in order to increase its sensitivity to rapid variations of incident radiant energy, a source of radiant heat waves and means for supporting the tourmaline plate in a position to cause the incident radiant heat waves to fall upon one iv the parallel flat faces at substantially QO-degree angle of incidence.

5. A system for receiving, detecting and indicating a beam of heat bearing rays comprising an extremely thin plate of tourmaline the thickness of which is parallel to the optical axis of the tourmaline material, means for supporting the tourmaline plate in a space substantially devoid of atmosphere whereby pyroelectric charges may develop upon. incidence of heat rays upon the tourmaline free from piezoelectric E. M. F35 occasioned by change in pneumatic pressure. the supporting means comprising a chamber impervious'to heat bearing rays except for a window whereby a beam may be intercepted and the tourmaline screened from random radiant energy from other directions, and means external to the window and in position in line with the window and the tourmaline plate to interrupt a beam of heat energy in a periodic manner to cause periodic pyroelectric charges to be produced at the faces of the tourmaline plate and means for indicating the condition of electrical charge stance having the property of delevoping electrical charges in response to changes in temperature and pneumatic pressure of the ambient at-- mosphere, conducting electrodes physically connected to the element and means adjacent the surface of the element and exposed to incoming radiant energy for causing at least a portion of the energy of a beam of heat-producing waves incident upon the element to be converted into to receive radiant energy and conducting electrode physically connected thereto to receive charges induced by change in the thermal condition of the material whereby the possible changes in pneumatic pressure to which the tourmaline is subject having been reduced to negligible degree the charges generated and impressed upon the electrodes are occasioned substantially exclusively by radiant energy reaching the pyroelectric element through the pervious window.

8. An evacuated housing having a window transparent to heat-bearing rays, 9. pyroelectric element within the housing, said pyroelectric element comprising a plate of crystalline material .0015 to .005 inch thick, two electrodes respec-' tively adjacent to portions of the element between which pyroelectric charges are developed and a coating of energy absorbing material overlying the outer surface of one of the electrodes and means for supporting the element to present the face of the coated electrode to the transparent -window whereby heat-bearing rays are enabled to rapidly heat the element and the element may rapidly radiate heat energy to-malntain its sensitivity as a pyroelectric element.

. LEON J. SIVIAN. 

